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Find the formula for the compound that contains 72.4% iron and 27.6% oxygen.
Take a 100 g sample which will give you
72.4 g Fe and 27.6g O.
Now convert g of each to moles. moles = grams/molar mass.
Find the ratio of the elements tlo each other using small whole numbers with the lowest being 1.00. The easy way to do this is to divide the smaller number by itself which assures you of 1.00 for that element. Divide the other number by the same small number and round both to even numbers BUT don't round too much (not over 0.1 or so). For example, if you get Fe1O1.5, this is a ratio of 2:3. If you obtain numbers that can'tg be rounded, multiply by whole numbers until you get numbers that can be rounded for both. Post your work if need additional help.